66 PART I. GENERAL ACCOUNT 



Another very characteristic modification of the peritoneal epithelium is 

 the pleating of the splanchnopleura which may be observed in the same 

 species and is probably to be found in other pogonophores also. The lateral 

 blood vessels become particularly numerous by the time the level of the zone 

 of thickened papillae is reached. They are supported by folds of the basement 

 membrane which run out as sheetlets joining it to the mesentery or to the 

 ventral blood vessel. These pleats are covered on both sides by a continuous 

 layer of hyaline peritoneal cells (Figs. 45, 69). The pleats anastomose with 

 one another in many places to produce irregular closed cavities lined with 

 peritoneum. One such enclosed pocket is regularly present a little in front 

 of the girdles situated between the dorsal and ventral blood vessels at the 

 point where the mesentery disappears, and it extends backwards to the hind 

 end of the body. The walls of this structure, which in view of its constancy I 

 have called the median coelomic sac, are formed of a delicate epithelium in 

 whose cells small round intensely staining granules may be observed. The 

 outline and diameter of this sac are very variable, changing in accordance 

 with shape and development of the surrounding peritoneal folds (Figs. 22, 

 46). To judge from the fact that the median sac is also present in Siboglimim 

 caulleryi, it may be present in all pogonophores (Ivanov, 1960a). 



The median coelomic sac widens out in the front part of the postannular 

 region and the splanchnopleural folds take on the appearance of massive 

 lappets and expand so much that they almost fill the entire body cavity. The 

 large cells which form these lappets are arranged in many layers and shaped 

 irregularly (Fig. 46). They are vacuolated and usually contain seemingly 

 excretory granules, yellowish or muddy brown in colour. The cells of the 

 somatopleura in this region also take on the same nature and in the latero- 

 ventral part of the trunk form a rather thick layer several cells deep (Fig. 46). 



In the hind end of the trunk the folds of the splanchnopleura diminish 

 considerably in size but the cells retain their peculiarities. 



The metasoma has a pair of strongly developed coelomoducts, which will 

 be described and discussed in connexion with the nature and functioning of 

 the genital system (p. 90). 



The excretory system 



The excretory function is carried out by the coelomoducts of the protosoma 

 and, in the mesosoma and metasoma, by the cells of the splanchnopleura 

 which seem to act as a kidney of accummulation; that is to say they act as 

 nephrocytes storing the excreta in an insoluble form. 



The single pair of protosomal coelomoducts are always well developed in 



